The separation of gas streams, most notably air, into constituent components such as oxygen, argon and nitrogen has been practiced for many years utilizing energy intensive processes for the recovery of various purities and volumes of gas product. Chemical and/or physical adsorption of individual gas components, cryogenic distillation of various gas components, and gas permeation through membrane media have all been practiced to recover components of gas streams and particularly to recover oxygen and/or nitrogen from air. These processes generally suffer from high utility costs and lack of continuous or convenient regeneration or operation.
For example, in conventional cryogenic air separation procedures, the single largest variable expense is the cost of power. In many cases, a reduction of as low as 0.5 to 1% in power consumption can offer significant economic advantage. Consequently, there is great interest in reducing specific power, that is, the power consumed per unit of product produced
Most high volume plants for producing nitrogen by cryogenic air separation techniques currently use a molecular sieve to remove water and trace carbon dioxide from air prior to cryogenic distillation in a cold box. In many cases, the molecular sieve drying step is preceded by mechanical refrigeration. Such pretreatment reduces the load on the molecular sieve drier by lowering its operating temperature and condensing away much of the water. In most cases, adsorbents such as molecular sieves exhibit higher capacities as their operating temperatures are decreased.
While the power consumption of the prerefrigeration unit is small compared to that of the overall plant, a savings ranging from about 1 to 2% of the total power draw could be realized if the refrigeration pretreatment step could be eliminated. Further, elimination of such pretreatment would obviate the need for a piece of rotating equipment, reduce consumption of possibly environmentally objectionable refrigerants, and improve the reliability of plant operation.
Unfortunately, elimination of the refrigeration pretreatment is not feasible. The resultant higher load on the molecular sieve driers would require a dramatic increase in their size and, more importantly, would require such an increase in the amount of regeneration gas required for the molecular sieve unit that any energy savings would be more than eliminated by increased regeneration energy costs, particularly in warmer climates.
Nevertheless, some effort has been directed at eliminating the refrigeration pretreatment step in lower recovery plants where the waste stream from the cold box is used to cool water which is in turn used to cool the feed air prior to the drier. In another approach, low cost refrigeration systems such as ammonia refrigeration or heat pumps have been suggested. In spite of such efforts, where high efficiency separation is required to produce high quality product at high recovery, there is no substitute for the refrigeration pretreatment step in low temperature gas separation systems.
Rice, et al., U.S. Pat. No. 4,783,201, disclose a process and apparatus for dehydrating gases. Gas mixtures, such as air, are contacted with an asymmetric membrane having transport selectivity for water vapor vs. the feed gas of at least about 1000, to permeate a majority of the water contained in the feed gas.